ASTR178 Other Worlds A/Prof. Orsola De Marco 9850 4241 [email protected] Assignment 2 • Friday September 3, 4PM posted on Blackboard • Deadline 2 weeks later in the boxes • Returned October 4, after term break The Moon Practical • If you have not done it, read the unit outline where there is a link to how to do the practical on line. In last class: the terrestrial planets I • A few more things about the Moon to finish last week’s plan. • Orbital properties of Mercury (M), Venus (V) and Mars (M). • Naked eye observations, early telescopes (and early theries) and new observations. • Spins and rotations and hot to measure them. • Mercury mystery! • The terrain of Venus and Mars • Plate tectonics on Venus and Mars? • Volcanos on Venus and Mars (and Earth!) • The atmospheres of Venus and Mars (and Earth!) • Seasons on Mars • Evolutions of the atmospheres In this class: the terrestrial planets II • Plate tectonics on Venus and Mars? • Volcanos on Venus and Mars (and Earth!) • The atmospheres of Venus and Mars (and Earth!) • Seasons on Mars • Evolutions of the atmospheres • Water on Mars • Life on Mars? Why is Mercury’s spin period exactly 2/3 of its orbital period? Tides again!!! Venus before the space age • Average Venus temperature with no atmosphere ~40 C. • Atmosphere was known – possibly keeps the planet at a different temperature… • Water Adams and Theodore Dunham (1932) saw CO2 in spectrum – greenhouse!! • Cloud of Venus reflect a lot (see high albedo) maybe that keeps the greenhouse in check. • Answers have to wait for space probes…. Venus from space Mariner 2 (1962) first successful mission to another planet: • It detects microwave radiation and finds Venus • Hot (>400 C) • and Dry Venera 7 (1970; Russian) first lander, confirms Venus as a desert searing world (this picture is actually from Venera 13) Mars before the space age Percival Lowell (American) 1855-1916 HST image Viking Orbiter image Cratered surface. Craters had escaped detection. Some of the surface of Mars must be very old. Mars changing colour due to winds, not seasonal vegetation changes! Topography of Venus (radar altimeter from Magellan) Topography of Mars (laser measurements from the Mars Global Surveyor) Highlands are old and cratered, lowlands are younger and not heavily crated. Venus’ surface • Venus surface has only 15% the number of craters as the lunar maria, indicating an age of only 500 Myr. • There is plenty of evidence for active volcanoes and tectonic activity, but that activity has stopped. • It looks like Venus goes through resurfacing episodes, or more likely, that the tectonic activity is on-going but is distributed evenly. • Venus has as much heat as Earth and should have tectonic activity. • Possibly its thinner crust means that the inner heat can break the surface more often and more evenly. We call this “flake” tectonics. • Venus tectonic activity is local, with local volcanos and local upwelling, but no large scale mountain ranges and ridges. Plate tectonics vs. “flake” tectonics • Mars has no earth-like plate tectonics. There are no features similar to ridges and ranges. • The crust is 40-70 km thick (on Earth it is 5-35 km). Too thick to allow plate tectonics. • It did have tectonic activity a long time ago: the Tahrsis rise was a large magma rise, which also cracked the crust and made the Valles Marineris. Press release 28 August 2010 from Mars Express (ESA) • Orcus Patera (defined but irregular volcanic craters) • Elliptical depression, 380 km long; 1800 m rims • Near Olympus mons • Created by oblique impact? • Shaped by plate tectonic (presence of “graben”)? Venusian volcanos • 10 million years old! • Present day volcanic activity • Sulfuric compounds in the atmosphere at the 0.015% level (which is high compared to Earth.) Volcanos on Mars (and Venus) cannot be caused by subduction. They must be caused by hot spots (like some volcanos on Earth – e.g., Hawaii). The lack of plate movement on Mars means that the volcano had time to grow huge. Most volcanos are old but Olympus Mons is very young, Why? Olympus Mons 24 km high – compare to Mona Loa (Hawaii) 8 km high Venera 13 (Soviet; 1981) one of the first landers Venera 13 measured a temperature high enough to melt lead and pressures of 90 bars! Comparison of the atmospheric structures of Earth, Venus and Mars Venus’ clouds have a 4 day rotation speed – the planet has a 243 day spin period. WHY? Let’s take a look at Mars’ atmosphere Residual ice cap may contain water ice. Very fine dust found by Viking 1 lander is easy to lift up and Create dust storms. Martian dust devils Caused by warm air rising and carrying dust up in the air. They are very large on Mars due to the fineness of the dust, and they can be seen from orbit. Outgassing of CO2 (H2O, N2 and SO2) from volcanos happened on Earth, Venus and Mars, originating thick atmospheres On Earth On Venus On Mars All terrestrial planets start with heavier atmospheres. Earth: liquid water modest CO2 Venus: initial H2O Greenhouse effect. More heat and no tectonics, so more CO2. Hence runaway greenhouse Mars: H2O froze and rained to the ground. Temperature dropped even more. CO2 remained partly in the atmosphere (not enough to warm). Key Ideas • Motions of Mercury, Venus, and Mars in the Earth’s Sky: Mercury and Venus can be seen in the morning or evening sky only, while it is possible to see Mars at any time of night depending on its position in its orbit. • At their greatest eastern and western elongations, Mercury is only 28° from the Sun and Venus is only 47° from the Sun. • The best Earth-based views of Mars are obtained at favorable oppositions, when Mars is simultaneously at opposition and near perihelion. Key Ideas • Rotation of Mercury, Venus, and Mars: Poor telescopic views of Mercury’s surface led to the mistaken impression that the planet always keeps the same face toward the Sun (1-to-1 spin-orbit coupling). • Radio and radar observations revealed that Mercury in fact has 3-to-2 spin-orbit coupling: The planet rotates on its axis three times every two orbits. • Venus rotates slowly in a retrograde direction. Its rotation period is longer than its orbital period. • Mars rotates at almost the same rate as the Earth, and its rotation axis is tilted by almost the same angle as the Earth’s axis. Key Ideas • Mercury’s Surface, Interior, and Magnetic Field: The Mercurian surface is pocked with craters, but there are extensive smooth plains between these craters. • Long cliffs called scarps meander across the surface of Mercury. These probably formed as the planet’s crust cooled, solidified, and shrank. • Mercury has an iron core with a diameter equal to about 3⁄4 of the planet’s diameter. By contrast, the diameter of the Earth’s core is only slightly more than 1⁄2 of Earth’s diameter. • Mercury has a weak magnetic field, which indicates that at least part of the iron core is liquid. Key Ideas • Comparing Venus and Mars: Most of the surface of Venus is at about the same elevation, with just a few elevated regions. On Mars, the southern highlands rise several kilometers above the northern lowlands. • Venus has a thick atmosphere and a volcanically active surface. Mars has a very thin atmosphere and little or no current volcanism. • There is no evidence of plate tectonics on either Venus or Mars. On Venus, there is vigorous convection in the planet’s interior, but the crust is too thin to move around in plates; instead, it wrinkles and flakes. On Mars, the planet’s smaller size means the crust has cooled and become too thick to undergo subduction. Key Ideas • Volcanoes on both Venus and Mars were produced by hot spots in the planet’s interior. • The entire Venusian surface is about 500 million years old and has relatively few craters. By contrast, most of the Martian surface is cratered and is probably billions of years old. The southern highlands on Mars are the most heavily cratered and hence the oldest part of the planet’s surface. Key Ideas • The Atmospheres of Venus and Mars: Both planetary atmospheres are over 95% carbon dioxide, with a small percentage of nitrogen. • The pressure at the surface of Venus is about 90 atmospheres. The greenhouse effect is very strong, which raises the surface temperature to 460°C. The pressure at the surface of Mars is only 0.006 atmosphere, and the greenhouse effect is very weak. • The permanent high-altitude clouds on Venus are made primarily of sulfuric acid. By contrast, the few clouds in the Martian atmosphere are composed of water ice and carbon dioxide ice. Key Ideas • The circulation of the Venusian atmosphere is dominated by two huge convection currents in the cloud layers, one in the northern hemisphere and one in the southern hemisphere. The upper cloud layers of the Venusian atmosphere move rapidly around the planet in a retrograde direction, with a period of only about 4 Earth days. • Weather on Mars is dominated by the north and south flow of carbon dioxide from pole to pole with the changing seasons. This can trigger planetwide dust storms. Key Ideas • Evolution of Atmospheres: Earth, Venus, and Mars all began with relatively thick atmospheres of carbon dioxide, water vapor, and sulfur dioxide. • On Earth, most of the carbon dioxide went into carbonate rocks and most of the water into the oceans. Ongoing plate tectonics recycles atmospheric gases through the crust. • On Venus, more intense sunlight and the absence of plate tectonics led to a thick carbon dioxide atmosphere and a runaway greenhouse effect. • On Mars, a runaway icehouse effect resulted from weaker sunlight and the absence of plate tectonics.